Protective relays are commonly used to protect electrical power distribution systems. A simple protective relay can be an electro-mechanical contact relay having an energizing circuit coupled to the load on an electric power distribution line. When the load on a portion of the system exceeds certain parameters, i.e., a fault is present, the relay is energized to operate a circuit breaker or the like coupled to the power distribution line to thereby isolate a portion of the electrical distribution system having the fault. The contacts of the relay can perform various functions such as tripping a circuit breaker, generating an alarm or providing a signal to another protective relay. For the most part, such simple relays only provide the contact operation as an output. They do not provide any other indication of the conditions which produced activity in the output contacts.
For instance, the principal of operation of a distance measuring relay is that the distance of a fault can be determined by a comparison of the complex impedance of the line derived from the current and voltage at the relay to a reach characteristic. If this comparison indicates that there is a fault within the protection zone of the relay, a trip contact is actuated. Accordingly, “intelligent relays” have been developed having microprocessors capable of evaluating complex impedance or other variables to protect the electrical distribution system by isolating faults and to transmit diagnostic data and the like to computers or other devices for evaluation and display purposes. Typically, substations in an electric power distribution system have many protective relays. The intelligent relays can be programmed to perform various protection functions. For instance, digital distance relays and overcurrent relays are in use. These relays are capable of providing a great deal of information. For instance, data indicating the location of the fault, and the current and voltage at the fault are available from intelligent distance relays. Similarly, current information is available from an intelligent overcurrent relay. This information is often transmitted to a remote computer for control and report generation. Conventionally, this is accomplished over an RS-232 channel or other serial communications channel for each relay. In addition to relays, other intelligent electronic devices, i.e. control devices having microprocessors capable of executing commands and/or collecting data, have been developed.
Accordingly, multiple serial port units are utilized to couple multitudes of intelligent electronic devices to remote a computer. Such units include multiple serial ports, 8 for example, and a single Ethernet port for coupling the serial ports to a network communications link. The Ethernet port is coupled to a computer using standard cables and protocols, e.g. a twisted pair. The serial ports are each connected to an intelligent electronic device. The multiple serial port unit uses known hardware and software to map the serial ports over the Ethernet connection to appear as local serial ports to the computer. Accordingly plural intelligent protective relays can be coupled to a computer over a great distance by an Ethernet connection.
However, intelligent electronic devices and associated computer systems are applied under hazardous conditions. Particularly, electrical distribution systems are inherently in environments that include temperature changes, high EMF, and other environmental hazards. Accordingly, computers, network connections, and other components often fail intermittently or permanently in electrical distribution systems. If a computer or network connection over which an intelligent protective relay is communicating fails, the operator may not have control of devices such as circuit breakers, disconnect switches, and the like. Also, the operator might not know the status of their devices and may lose centralized control of an entire substation. Accordingly, the protective relay may not function properly and equipment damage or injury of personnel may result. Accordingly, there is a need for failure tolerance in a control system for protective relays in an electric power distribution system.
It is known to provide network failure tolerance using redundant computers i.e., servers, and to switch over to a backup computer when a primary computer fails. Also, it is known to send separate packets of data over different paths in an Ethernet connection. However, these systems and methods are not truly redundant because they rely on the computers for fault detection and a single Ethernet connection for communications.